The sardine run occurs most years between May and July when billions of sardines (Southern African pilchard Sardinops sagax) spawn in the cool waters of the Agulhas Bank then and move along the southern coast of South Africa, eastward at first, then northward as they follow the coastline. This migration is followed by large numbers of predators, which provide the main attraction for divers.

The run, containing millions of individual sardines, occurs when during the winter months a cold south to north-flowing current develops off the east coast, moving inshore and counter to the Aghulas current. A part of the sardine population follows this narrow band of cool water north to Port Edward, swimming up between the coast and warm Aghulas current. The numbers vary from year to year, and it is only considered a "run" when the shoals are big enough to be visible at the surface.

North of Port Edward the cold current is restricted by the narrowing continental shelf and the shoals become concentrated in a narrow inshore band of water, as far as Mozambique where it then leaves the coastline and goes further east into the Indian Ocean.

The shoals may be more than 7 km long, 1.5 km wide and 30 meters deep and are clearly visible from spotter planes or from the surface.

Sardines group more closely together when they are threatened. This instinctual behaviour is a defense mechanism, as lone individuals are more likely to be eaten than large groups, but this behaviour also encourages large numbers of predators to follow the shoals.

Thousands of dolphins are largely responsible for rounding up the sardines into bait balls. These bait balls are densely packed masses of fish and can be 10–20 metres in diameter and extend to a depth of 10 metres and may last for up to about 10 minutes. Once the sardines are rounded up, sharks , game fish birds and the occasional whale take advantage of the opportunity. This is the most desirable time for a diver to join the action.

Spotter aircraft are used by some operators to pinpoint the action. Others rely on luck and following the ones who use spotter planes.

The annual northward coastal migration of Humpback whales occurs at much the same time, and may. if you are lucky, provide extra entertainment.

Divers from all over the world travel to South Africa for the sardine run, but it is not known for a reliable schedule, and there is a great deal of waiting and some luck involved. A small but significant percentage of visitors may not get to see the sardines and associated predators at all due to bad luck and unsuitable weather. Be aware of this when you book.

The sardine run is still poorly understood from an ecological point of view. There have been various hypotheses, sometimes contradictory, that try to explain why and how the run occurs.

A recent interpretation of the causes is that the sardine run is most likely a seasonal reproductive migration of a genetically distinct subpopulation of sardine that moves along the coast from the eastern Agulhas Bank to the coast of KwaZulu-Natal in most years if not necessarily every year.

The migration is restricted to the inshore waters by the preference of sardine for water temperature below 20°C, and the strong and warm offshore Agulhas Current, which flows in the opposite direction to the migration, and is strongest just off the continental shelf. A band of cooler coastal water with reduced or reversed current makes it possible for sardine shoals to swim north.

Decreasing sea surface temperature. This is most usefully available as sattellite data, as it covers the whole area.

Calm current conditions.

Light north-westerly land breezes

Stable atmospheric conditions.

Unfavourable:

Increasing sea surface temperature.

Moderate north to south currents.

Large swells.

Turbid water. These last two will usually make the boating and diving less pleasant anyway, and the less enthusiastic divers may well take the day off for less strenuous activity.

North-easterly and north-westerly winds and north to south currents have a cooling effect upon nearshore sea surface temperatures, but south-easterly winds and increasing air temperatures cause nearshore sea surface temperature warming.

Predators as predictors:

The Cape gannet is the predator species most closely associated with sardine presence along the Eastern Cape and KZN coastline and is the most useful indicator of sardine run activity.

Sharks and large gamefish presence is also strongly associated with sardine presence during the run, but as they are not as easily observed from the surface they are not as useful a predictor of run activity

The presence of common dolphins inshore along the east coast during winter is significantly associated with sardine presence and common dolphin can be considered the third most useful species for predicting sardine presence.

The resident poulation of bottlenose dolphin does not appear to associate with the sardine run, whereas the migrant stock does. This may explain why the bottlenose dolphin is less likely to predict sardine presence.

Sharks that follow the sardine run are primarily the bronze whaler, but also dusky shark, ragged tooth shark, blacktip shark, spinner shark and zambezi shark. Whale sharks and great whites have also occasionally been seen.

Seabirds include the Cape gannet, cormorants, terns and gulls and the occasional penguin, albatross, or petrel.

Marine mammals include Bryde's whales, Humpback whales, dwarf minke whales, common and bottlenose dolphins (estimates of about 18000 dolphins). The Cape Fur Seal follows the shoals as far as Port St Johns. Southern right whales have also been seen, but may not be directly associated with the sardine run, and occasionally Orca have been spotted.

The sardine hotline on 082 284 9495 gives regular reports and updates on the shoal's progress up the coast.

If you will be using your own Scuba set, it is probably better to take a couple of small cylinders than a big one, as you may be climbing in and out of the boat several times as the action moves around. Many photographers will even freedive to keep mobile, but this limits underwater time.

Get fit and practice buoyancy skills. This is all mid-water diving, usually without marker bouys, and usually without a dive leader to show you around. The action is fast and moves around a lot. Some operators require minimum certification of Advanced Diver, and require certification card and log book to be shown as evidence of competence. Soime operators also require a certificate of medical fitness to dive.

Good fins and good finning fitness will help you keep up. This is scuba diving where long fins are an advantage if your legs are up to it.

The sardine run is in winter, air temperatures can be expected to range between 15°C and 24°C, and rain is possible. Sea conditions may vary between flat and windless and roller coaster swell with a stong wind.

Keep drag to a minimum. The water is normally warm enough (around 15°C to 21°C) for 5mm wet suits wth boots gloves and hood. People who get cold easily might prefer a 7mm semi-dry. Avoid bulky BC jackets, dangling hoses and anything else that sticks out to the sides.

It is worth taking an underwater camera or video recorder if you have one. Know how to use it before you dive, and set up for natural light wide angle unless you are quite proficient. If you have no camera, consider getting a GoPro headmount video, but if you do, also get the flat lens port adaptor. Put this on your head and it will record everything you look at during the dive. Quality may not be wonderful, but you will have the record.

Visibility may range between 2m and 20m. Some operators will not allow diving unless visibility exceeds 5m for safety reasons.

For the boat trip, you should take sun protection, a hat, sunglasses and perhaps binoculars.

Bookings for boat space are usually made well ahead of time. Don't expect to turn up and get a place on a few hours notice. Book early, and have a flexible schedule Mdash; the sardines are not concerned with your convenience, and will move at their own speed.

Charter boats from all over the country may follow the sardine run, not only those from the area the fish are passing.

If you are visiting South Africa specifically to dive the Sardine run, you would probably enter at Johannesburg and take a connecting flight to Durban, which has the nearest major airport to the KZN south coast.

The diving is from boats. You will meet your charter boat at the launch site, and travel out to the shoal, where you hope for a bait ball. Depending on the charter conditions, you may free dive or scuba dive with the shoal and the predators.

The sardine run is not considered a high risk diving activity, but there are some skills which are necessary for your safety. Probably the most important is good buoyancy skill, as you will be diving at least part of the time over deep water, though the action is mostly near the surface.